A cup of coffee, tea, or a cola drink contain enough caffeine to allow the analysis of caffeine metabolites in urine; different ratios of selected metabolites yield information on the activity of AHH (a liver enzyme known to activate many carcinogens), and classify each subject by his or her genetically controlled acetylation capacity (a capacity which seems to affect susceptibility to bladder cancer). Some limits of error of the test remain to be defined; then, its ranges of usefulness will be established in collaboration with NIOSH, by testing two cohort of industrially exposed subjects. One of these cohorts consists of about 250 subjects for whom blood dioxin levels and extensive medical, demographic, and life- style data are known to NIOSH. Some individuals have very high body burdens of dioxin. The other cohort consists of 1200 workers exposed to aryl amines. Some individuals have developed bladder cancer. NIOSH will select and contact subjects for testing and send their urine specimens for analysis to Toronto. In this collaborative study, the focus of interest of the Toronto investigators is in establishing the usefulness of caffeine metabolic ratios as indices of toxic exposure and susceptibility. NIOSH is primarily interested in obtaining information by non- invasive methods which should help in answering epidemiological questions of medical concern.